Grunt minors were the weakest and least skilled grunts in the Covenant army. They are considered recruits. They run away if you killed the majority of the grunts around them just like Grunt majors.
History
Their ancestors invented the Needler. They were much amarter than they are now. They started to dumb down after the Grunt Rebellion.The elites thought if the grunts were stupid, that another Grunt Rebellion will not happen again.